Electrolytic cell and method of lining the same



' April 21, 1925. 1,534,322

W. HOOPES ET AL ELECTROLYTIC CELL 1ND HETHOD OF LINING THE SAME Filed D80. 21, 1922 hoentoz;

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- a. .Honsnszo Mum-,- aumqi lil lit-3 Patented Apr. 21, 1925.

units!) STATES WILLIAM acorns,

ALUMINUM COMPANY OF AMERICA, POLRATIUN OF PENNSYLVANIA.

PATENT OFFICE.

orrrr'rsn'oaen, JUNIUS 1). EDWARDS, or camera, PENNSYL- vnn'ra, am) BASIL a. HOBSFIELD, or

BADIN, NORTH CAROLINA, ASSIGNORS TO 01 PITTSBURGH, PENNSYLVANIA, A; COR- ELECTROLYTIC CELL AND METHOD OF LINING THE SAME.

Application filed December 21, 1922. Serial 1T0. 608,289.

To all whom it may concern:

Be it known that we, lV1LL1AM Hoorns, .lUNIUs D. Eownnns, and Basin T. Hons- FIELD, all citizens of the United States of ilimerica, the said l VILLIAM Hoorns residing at Pittsburgh, in the county of Allegheny and State of Pennsylvania, the said JUNIUs l). Eowanos residing at Oakmont, in the county of Allegheny and State of Pennsyl- Vania and the said BASIL T. HORSFIELD residing at Badin, in the county of Stanly and State of North Carolina, have invented certain new and useful Improvements in Electrolytic Cells and Methods of Lining the Same, of which the following is a full, clear, and exact description. I

This invention relates to electrolytic refining of metals, more particularly to the cell or pot in which the refining process is carried on. in the copending applications of William Hoopes, Serial Nos. 608,287, and 608,288 both tiled December 21, 1922, there are described and claimed broadly an electrolytic cell and a method of providing the same with a thermally and electrically insulating side-lining, in which a side-lining oi that character is formed by freezing on the inner surface of a metal shell a molten mixture of .suitable materials, preferably containing aluminum and sodium fluorids and one or more other fluorids, for example barium or calcium fiuorid or both, capable (at least when present in substantial amount) of raising the freezing point of the mixture. The present invention relates to similar apparatus and methods, and its chief object brieflystated, is to provide a cell or pot having a lining, especially a side-lining, possessing in a high degree the properties of thermal and electrical insulation and of resistance to fusion. To this and other ends the invention consists in the novel features of apparatus and method hereinafter described.

The method herein described and claimed is applicable to a variety of electrolytic cells and to other containers in which a lining of the character indicated is necessary or desirable. For electrolytic refining of aluminum a cell of the general type described in the aforesaid applications, of William Hoopes is particularly advantageous, and

such a cell is illustrated in the accompanying drawing, in which the cell is shown in cross section.

The apparatus illustrated comprises a lower steel shell 10 of cylindrical form and an upper steel shell 11, the two constituting the lower and upper sections of the com plete shell. The shell sections are secured together in any convenient manner, and are electrically separated or insulated from each other as indicated by the insulatin ring or gasket 12. They are also provi ed with cooling means, as for example annular water jackets 13, 14, in the neighborhood of the joint between the two sections. Cooling water is supplied to the lower water jacket through a pipe and inlet nipple 16 and flows in both directions to an outlet nipple 17. Thence it passes through pipe 18 and inlet nipple 19 to the upper water jacket, in which it flows in both directions to the outlet nipple 20 and thence through ipe 21 to the waste pi e 22. To prevent e ectrical grounding, rub r hose may be used for the pipes 15 and 21, and rubber hose may be used for the pipe 18 to prevent material flow of current from one cell section to the other. lit will be understood that the water used should be sutliciently pure to be substantially non-conducting at the voltage employed in the cell. Tn the bottom is a layer of heat insulating material 23, and on this is a bottom lining 24 of conducting material, as for example carbon. Good electrical connection is afforded by metal collector plates 25 welded to the inside of the lower shell section and embedded in the carbon bottom.

Electrical connection between the lower shell and the external c1rcu1t or source of current, not shown, may be made by means of a terminal or busbar 26 welded or bolted to the shell. The carbon bottom, which constitutes the lower electrode of the cell, is formed with a cavity or depression in its top, as indicated, and with a tapping hole 27, which may be closed by a plug 28 of suitable refractory material, as for" example a dense charcoal plug. The upper electrode maybe of multiple form, comprising a suitable number of carbonaceous cylinders or rods 29 in the upper ends of which the copper or other metal rods 30 are secured.

Electrical connection may be made between these metal rods and the external circuit or source of current by means of a terminal or busbar 31 to which the rods are clamped or otherwise secured in any convenient and suitable manner, not shown.

On the sides of the cell is a thermally and electrically insulating lining .32, in close union with the carbon bottom 24 and extending over the joint between the shell. sections to a plane well up in the upper section. Inaccordance with the referred practice of our resent invention or use in electrolytic refining of aluminum, this sidelining has, as an essential ingredient, alum:

ina in corundum-like form; and is formed by freezing a crust on the sides of the cell, from a molten mixture having 'a composition (and containing alumina) adapted to cause selective crystallization upon the cell walls of a layer of different composition and containin a substantial amount of alumina. Preferably the mixture contains aluminum, barium and sodium fluorids.

The following is a convenient and effective method of forming the side lining in a cell of the type illustrated:

The upper electrodes (which for this purose may be composed of amorphous caron) are lowered into contact with the carbon bottom and current is sent through the cell from the upper electrodes to the lower, resulting in generation of heat at the points of contact. Powdered or granular bath mixture, preferably with a suitable amount of a high melting substance, is deposited in the cell, and after a sufficient quantity has been melted the upper electrodes are raised so that the current is compelled to pass through the fused bath. Molten metal or alloy of suitable character, heavier than the bath, is now poured into the bottom of the cell, thereby raising the bath so thatthe upper electrodes must be raised farther to keep their lower ends out of contact with the metal, and more bath material is sup lied until the cell is filled to the desired helght. The bath should, however, remain in contact with the carbon bottom-lining at the edges of the cavity therein, and if the cell is intended for use in refining aluminum the metal employed to raise the bath is preferably an aluminum alloy in composition the same as or similar to that which is to be refined in the subsequent refining operation of the cell. In the meantime Wateris circulated through the water jackets, and the high melting constituent in powdered or granular form is fed to the bath, preferably in amount sufiicient to keep the bath saturated therewith. Under such conditions there is formed on the cooled portions of the shell sections, by selective crystallization thereon, a hard, firmly adherent crust of substantial thickness, having a different composition from that of the fused or liquid bath and containing a large amount of the high melting ingredient in coarsely crystalline form. A crust thus produced from a fiuorid mixture containing alumina as the chief high melting constituent has a freezing point such that temperatures as high as 1050 C. can subsequently be used in the cell without fusing the crust, and even higher temperatures if the crust is cooled, as by means of the water jackets. Nor is the crust attacked in any substantial degree in subsequent use of the cell with a bath composed of fused fluorids; and since the alumina crystals are large they suffer but little by the solvent action of such baths, whereas in crusts formed from unsaturated mixtures the alumina is deposited in the crust in the form of extremely fine particles which are readily redissolved.

The mechanism of the formation of the above described side-lining crust by our improved method appears to be as follows:

Alumina becomes more soluble in cryolite baths as their temperature is raised, but if alumina be added until the bath is approximately saturated, it will be found that a small drop in temperature will cause some of the alumina to precipitate out as corundum or in corundum-like form, with which more or less bath will be'mechanically associated.

In an operating cell the portion of the bath adjacent to the walls is at a temperature distinctly lower than that of the main body of the bath, so that if sufficient alumina is employed to saturate this main body the natural circulation will cause a gradual deposition of part of the alumina on the walls of the cell inthe form of a thickened crust. By reason of their moderate rate of growth, the alumina crystals in this crust tend to attain a relatively considerable size, so that the crust when broken shows a coarsely crystalline structure.

After the crust has been built up to a suitable thickness, its formation may be arrested by reducing the alumina content of the bath. This may be done, for example, by removing art or all of the bath and replacing it with fresh bath of similar or suitable composition, but subsiautially free from alumina. Another and more convenient means consists in stopping the addition of alumina to the bath and electrolyzing the latter, using a carbon anode and a heavy molten alloy cathode, and thus deoxidizing the bath by electrolytic reduction of alumina.- 'When the bath has been brought to a satisfactory condition, the cell may be put in operation, for electrolytic refining, by the method described in the aforesaid applications of William Hoopes. The crust formation can also be arrested by breaking the connection with the source of crust may be composed current and removing the molten bath, as by di ping it out.

or aluminum refinin the side-lining or formed from a mixture containing alumina and aluminum fluorid and one or more alkali metal or alkalin-earth metal fluoride, as for example dition of barium, sodium, and strontium. The presence of chlorids of the metals mentioned is permissible but in general is not desirable. Calcium and ma esium fluorids are also permissible, A mixture composed of or containin by weight, about 9 parts aluminum iluori 9 parts barium fluorid, and 7 parts sodium fluorid, with enough alumina to saturate the mixture, has been found eminently satisfactory and so has also (with the adalumina) the bath or electrolyte mixture describedin a copending application of William Hoopes, Francis C. Frary, and J unius D. Edwards.

The voltage and amperage needed to fuse the mixture depend largely upon its composition and the quantity to be melted, the size of the cell and theefi'ectiveness of its heat-insulation, etc., and hence it is impos sible to give definite figures which would be density of universally applicable. In practice the power input is regulated to produce fusion of the materials and, maintain them at the desired temperature. With a cell constructed as and in about the proportions indicated in the accompanying drawing, a voltage between 7 and 9 volts and a current 800 amperes per square foot were found satisfactory for use with the fluorid and alumina mixture stated quantitatively. above.

The preferred method of forming the boundary crust or side-lining has been described above. It is, as already intimated, highly desirable that this crust should be made as non-conductive as possible, both electrically and thermally, since thermal conductivity permits excessive loss of heat to the water jacket, thus increasing the power consumption required to maintain the operating temperature, and electrical conductivity permits current to pass from, anode to cathode without depositing metal on the latter, thus decreasing the current eiliciency. The crust is composed of frozen materials with considerable portions of alumina in corundum-like form. These materials when cold, it uncontaminated by impurities, have 'the desired electrically and thermally insulatin properties but they become more or less eectrically conductive when hot. It is important, in formingghe crust, to exercise care to maintain the th from which the crust is formed, substantially free from carhon and finely divided metal, which, if present, tend to be included in the crust and render it conductive at the time of formation. Covering the carbon bottom of the cell to be negative in .William Hoopes an Francis ,missible as graphite cylinders" and the carbon bottom- 1 with a. layer of metal as soon as possible in order to minimize the amount of carbon exposed, aids in maintainin the bath in a carbon-free condition. Having the top part of the shell electrically neutral during the crust-forming stage is also very advantageous. If this part ofthe shell is allowed the stage referred to, metalilic reduction products, especially sodium, tend to accumulate in the crust and render it conductive. If, on the other hand, it is allowed to be positive, iron tends to dissolve and contaminate the bath, and the resultant corrosion may cause penetration of the water jacket.

We have described above the formation of our electrically and thermally insulating rcfractory side lining, 111 a cell which is just being started. During the operation of the cell, it is found thatoxygen is'introduced into the bath, resulting in the formation of alumina therein, due to a number of causes; among which may be especially mentioned the cathodic production of sodium, its oxidation by the air, and the return of the resulting products to the molten bath. Unless suitable measures are taken to remove alumina from the bath electrol tically or otherwise, its concentration wi l gradually increase, and soon the deposition of alumina upon the insulating lining, and the consequent thickening thereof, will begin. When this lining reaches an undesirable thickness, its inner surface will have become so hot, due to the lessened flow of heat to the water jacket, that the concentration of alumina in uilibrium with it in the bath will be relatively high. If this crust be now broken out and removed, a new crust containing alumina in corundum-like form will be promptly reduced from the bath and the alumina which it contains, and the concentration of the latter in the bath will be corresponding itself from time to time, and thus the crust rich in alumina serves not only as an excellent insulating lining, but as a convenient means of removing excess alumina from the bath, in the method described and claimed broadly in the co ending agplication of Frary, Serial No. 608,286 filed December 21, 1922.

It is recognized-that, strictly speaking, the aluminum lying floating on the bath and the layer of alloy underlying the bath, are the upper and lower electrodes, respectively, but these layers are termed herein the cathode and the anode, and hence it is deemed perwell as convenient to refer to'the lining, or t eir equivalents, as the upper and lower electrodes.

It is to be understood that the invention is not limited to the apparatus and procedure herein specifically illustrated or described but can be carried out in other ways without departure from its spirit.

We claim 1. The process of producing a refractory insulating lining for an electrolytic cell, comprising freezing from a fused bath therein a lining containing the constituents of the bath in substantally different proportions from those in the liquid and substantially more refractory than a lining having the same composition as the liquid.

2. The process of producing a refractor insulating lining for an electrolytic cel comprising compounding and fusing a bath adapted for selective crystallization, upon the walls of the cell, of a .lining containing the constituents of the bath in substantially different proportions from those in the liquid and substantially more refractory than a lining having the same composition as the liquid; and cooling the cell walls to cause such selective crystallization thereon.

3. The process of producing a refractory insulating lining for an electrolytic cell, comprising freezing from a fused bath a lining containing a high-melting bath constituent of limited solubility and forming thereby a more refractory lining than one igavfing the same composition as the fused 4. The process of producing a refractory insulating lining for an electrolytic cell,

a constituentiof the bat comprising compounding and fusing a bath adapted for selective crystallization, upon the Walls of the cell, of a lini containing a high-melting constitutent of limited solubility, and producing thereby upon said walls a lining more refractory than one having the same composition as the fused bath.

5. The method of lining an electrolytic cell for the electrolysis of fused salts which consists in establishing in the cell a fused bath otherwise suitable to act as an electrolyte in the use of the cell but charged with a refractorymaterial desired as a constituent of saidlining, and depositing from the mixture a crystalline crust containing said material on desired portions of the cell walls.

6. The method of lining an electrolytic cell for the electrolysis of fused salts which consists in establishng in the cell a bath of fused material suitable to act as the electrolyte, charging said bath with a material more refractory than the normal electrolyte and suitable as a constituent of said lining, and cooling portions of the said walls whereby an adherent crystalline crust containing said material is deposited thereon from said bath.

The process of producing a refractorv insulating lining for an electrolytic cell,

comprising freezing from a fused bath in the cell, by cooling the walls of the cell, a lining containing a high-meltin material which is but which is of limited solubility therein,

' coarse? and producing thereby a more refractory. lining than one iiaving the same composition as the fused 8. In the electrolyticrefining of aluminum, the process of producin a refractory insulating lining for the refining cell, comprising freezing a lining upon the walls of the cell from a fused bath therein containing alumina and having a composition adapted to cause selective crystallization upon said walls of a layer rich in alumina and substantially more refractory than a layer having the same composition as the fused bath.

9. In the electrolytic refining of aluminum, the process of producing a refractory insulating linin for the refining cell, comprising establishing in the cell a fused fluorid bath containing alumina and having a composition adaptcd to selectively crystallize upon the walls of the cell a-layer containing suflicient alumina to render more refractory than a lining having the same composition as the fused bath, and frlelezing such a layer upon the walls of the ce 10. In the art of lining a cell or vessel for electrolytic refining and for other purposes, the improvement comprising freezing a coating on the interior surface of the cell out of a fused mixture containing one or more metal fluorids and. alumina in amount sufficient to substantially saturate the mixture.-

11. In the art of lining a cell or vessel for electrolytic refining and.- for other purposes,

it substantally more metal fluorids and alumina, and freezing from such mixture on the sides of the shell a crust or lining containing alumina in coarsely crystalline corundum-like form. I

12. In the art of lining a cell or vessel for electrolytic refining and for other purposes, the improvement comprising freezin a coating on the interior surface of the ce out of a fused mixture fluorid and alumina, the latter being in amount sufficient to substantially saturate the mixture.

13. In the art of lining a cell or vessel for electrolytic refining and for other purposes, the improvement comprising applying to the inner surface of a metal shell a molten mix' ture containing about 9 parts of aluminum fluorid,'9 parts of barium fluorid, 7 artsof sodium fluorid, and alumina, and f reezin from such mixture on the sides of the shefi a crust or lining containing alumina in crystalline corundum-like form.

14; n the art of lining a cell or vessel for electrolytic refining and'forother purposes, the improvement comprising fusing in the cell a mixture containing in solid form one or more metal fluorids and alumina" and containing one or containing aluminum cell a 'xture passing current through the fused portion of such mixture to continue fusion of the same, and freezing from the mixture on the inside of the cell a lining containing alumina in coarsely crystalline corundum-like form.

15. In the art of lining a cell or vessel for electrolytic refining and for other purposes,

the improvement comprising generating heat in the cell by electrical energy an a mixture containing aluminum, barium, and sodium fiuorids, and alumina, and freezing from the fused mixture on the interior surface of the cell a crust having a'freezin point higher than that of the same fiuorids when unsaturated with alumina.

16.-In the art of lininga cell or vessel for electrolytic refining and for other purposes, the improvement comprising generating heat in the cell by electrical energy and fusing by such heat a mixture containing about 9 parts of aluminum fluorid, 9 parts of fluorid, and alumina, and frcezin from the fused mixture on the interior surl acc of the cell a crust or lining containing alumina in coarsely crystalline corundum-like form.

17. n the art of lining a cell or vessel for electrolytic refining and for other purposes,

the inproveme'nt comprising fusing in the containing one or more metal lluorids and freezing on the inner surface of the cell, while supplying alumina to the fused mixture, a crust or layer containing alumina in corundum-like form.

18. In the electrolysis of a fused bath in a cell having a lining composed of bath ingredicnts in solid form, the steps comprising accumulating in the bath a high-melting ingredient thereof, removing more or less ofthe lining, and selectively crystallizing1 or;

the cell walls a solid lining compose bath ingredients but containing said highmeltin ingredient in excess.

19. n the electrolysis of a fusedbath containing a high-melting ingredient of limited solubi ity, in a cellhaving a lining composed of bath ingredients in solid form, the steps ingJ'and for other purposes, comp comprising removin more or less of the lining, and selective y crystallizing on the cell walls a lining of bath ingredients with said hi h-melting ingredient in a proportion adapted to make the lining more refractory than one having the same composition as the remaining hath.

20. In the electrolytic refining of aluminum with a fused bath in a cell having a lining composed of bath ingredients in solid form, the steps comprising removing more or less of the lining, and selectivel crystallizing thereon a lining containing a umina in higher proportion than in the liquid.

21. In the electrolysis of a fused bath in a cell having a lining composed of bath ingreilients in solid form, the steps comprising removing more or less of the lining, and cooling the cell walls to-selectively crystallize thereon from the batha solid layer containing bath ingredients in substantially diil'erent proportions and substantially more. refractorv than a lining havin the same composition as the rcmainin bat A cell or vessel for e ectrolytic refining and for other urposes, having a thermally and electrica y insulating lining composed of one or more metal fluorids and alumina in coarsely crystalline corundum-like form.

v 23. A cell or vessel for electrolytic refining and for other purposes, comprising a metal shell having a limn com osed essentially of one or more meta fluoride and alurinina in coarsely crystalline corundum-hke 'orm.

24. A cell or vessel for electrolytic refin rising metal shell having an interior side-lining composed of one-or more metal fluorids and crystalline corundune.

alumina in coarsely like form.

In testimony'whereoi we hereto afiix our signatures. 1

WILLIAM HOOPES. JUNIUS .D. EDWARDS. BASIL T. HORSFIELD.

containing alumina 

